1. Field of the Invention
This invention relates to an image taking lens system, and particularly to a so-called macrolens capable of photographing an infinity object to a short-distance object of a one-to-one magnification degree in a silver halide photographic camera, a video camera, a digital still camera or the like.
2. Related Background Art
As an image taking lens chiefly directed to the photographing of a short-distance object, there has heretofore been what is called a macrolens or a microlens (hereinafter referred to as the “macrolens”).
The macrolens, as compared with other image taking lenses such as an ordinary standard lens and a telephoto lens, is designed such that high optical performance is obtained particularly in case of the photographing of a short-distance object.
Also, in many cases, the macrolens is utilized for the photographing of objects at a wide range of distance including not only a short-distance object but also an infinity object.
Generally, in case of focusing from an infinity object to a short-distance object, as the photographing magnification becomes greater, the fluctuations of various aberrations become vehement and optical performance is aggravated. Therefore, a floating method has heretofore been adopted to correct the fluctuations of the various aberrations.
In Japanese Patent Application Laid-Open No. 2-19814 (corresponding U.S. Pat. No. 4,986,643) and Japanese Patent Application Laid-Open No. 2-285313 (corresponding U.S. Pat. No. 5,007,720), there is disclosed a lens system capable of short-distance photographing which is comprised of three lense units, i.e., a first lens unit of positive refractive power, a second lens unit of positive refractive power and a third lens unit of negative refractive power. In these examples of the prior art, there is proposed a focusing method whereby when effecting photographing at a low magnification to a high magnification in conformity with a change in an object distance, floating is effected with the first lens unit of positive refractive power and the second lens unit of positive refractive power moved to the object side while the interval between the first lens unit and the second lens unit is changed with the third lens unit of negative refractive power kept stationary relative to the image plane.
However, in the method whereby the first lens unit is moved in case of floating, the amount of movement of the lens unit generally becomes great. Therefore, to adopt this method in a camera having the auto focusing function, an actuator having great driving torque becomes necessary, or otherwise high-speed auto focusing becomes difficult.